Portable medical devices are useful for patients that have conditions that must be monitored on a continuous or frequent basis. For example, diabetics are usually required to modify and monitor their daily lifestyle to keep their blood glucose (BG) in balance. Individuals with Type 1 diabetes and some individuals with Type 2 diabetes use insulin to control their BG levels. To do so, diabetics routinely keep strict schedules, including ingesting timely nutritious meals, partaking in exercise, monitoring BG levels daily, and adjusting and administering insulin dosages accordingly.
The prior art includes a number of fluid infusion devices and insulin pump systems that are designed to deliver accurate and measured doses of insulin via infusion sets (an infusion set delivers the insulin through a small diameter tube that terminates at, e.g., a cannula inserted under the patient's skin). In lieu of a syringe, the patient can simply activate the insulin pump to administer an insulin bolus as needed, for example, in response to the patient's high BG level.
A typical infusion pump includes a housing, which encloses a pump drive system, a fluid containment assembly, an electronics system, and a power supply. The pump drive system typically includes a small motor (DC, stepper, solenoid, or other varieties) and drive train components such as gears, screws, and levers that convert rotational motor motion to a translational displacement of a stopper in a reservoir. The fluid containment assembly typically includes the reservoir with the stopper, tubing, and a catheter or infusion set to create a fluid path for carrying medication from the reservoir to the body of a user. The electronics system regulates power from the power supply to the motor. The electronics system may include programmable controls to operate the motor continuously or at periodic intervals to obtain a closely controlled and accurate delivery of the medication over an extended period.
Some fluid infusion devices use sensors and alarm features designed to detect and indicate certain operating conditions, such as non-delivery of the medication to the patient due to a fluid path occlusion. In this regard, a force sensor can be used in a fluid infusion device to detect when the force applied to the fluid reservoir stopper reaches a set point. The force sensor in such a fluid infusion device could be positioned at the end of the drive motor assembly that actuates a rotatable lead screw, which in turn advances the stopper of the reservoir. With such an arrangement, the force applied to the force sensor by the drive motor assembly is proportional to the pressure applied to the medication as a result of power supplied to the drive system to advance the stopper. Thus, when a certain force threshold (a set point corresponding to an occlusion condition) is reached, the fluid infusion device is triggered to generate an alarm to warn the user.
Early detection of an occlusion condition is helpful, because an occlusion can result in “under-dosing,” particularly if the drive system continues to receive commands to deliver medication when the fluid path is blocked. Accordingly, proper operation of the force sensor is important for purposes of occlusion detection, and it is desirable to have some diagnostic capability related to the health of the force sensor.
Existing force-based occlusion detection techniques typically rely on a fixed threshold or set point that is indicative of an occlusion condition. A threshold value is selected based on system tolerances. To avoid frequent false alarms, however, it is necessary to set the threshold value above the maximum expected force, based on the interacting system components. Because the threshold value is set at the maximum expected force, if a patient has a particular pump system with a nominal delivery force, it may take slightly longer to reach the threshold force. Accordingly, it is desirable to have an occlusion detection technique that does not solely rely on a fixed occlusion detection threshold force.
Some fluid infusion devices use replaceable fluid reservoirs that are secured in the housing of the device and actuated by a drive assembly. One form of infusion pump utilizes a threaded cap to seat and secure the fluid reservoir in the housing of the pump. The user unscrews the threaded cap to remove an empty reservoir, replaces the old reservoir with a new reservoir, and reinstalls the threaded cap to secure the new reservoir in place. During use, the threaded cap might be dislodged (especially if the fluid infusion device is a portable unit that is worn by the patient), resulting in an unseated or improperly installed reservoir. For example, if the user participates in certain physical activities (e.g., sports, hiking, or rigorous exercise), then the cap might be unintentionally loosened by physical rotation. As another example, if the user is in a crowded environment (e.g., a concert, a nightclub, or a full elevator), then the cap might be inadvertently unscrewed through contact with another person or an object. For this reason, it is desirable to have a reservoir presence and/or seating detection technique for a fluid infusion pump.